The invention relates to detection of synchronizing signals in the digital data received by digital television receivers, for acquiring data field and line synchronization, and for acquiring symbol synchronization.
Vestigial sideband (VSB) signals that are used in terrestrial through-the-air transmissions of digital high-definition television (HDTV) signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This percentage modulation has been standardized as being ⅝ times as large as the smallest change in symbol code level in symbol codes having eight levels. Such VSB signals have been chosen for over-the-air broadcasting within the United State and can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than VSB signals.
Radio receivers for receiving digital television signals, in which receiver the final intermediate-frequency signal is somewhere in the 1-8 MHz frequency range rather than at baseband, are described by C. B. Patel et alii in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER, and included herein by reference. The use of infinite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,548,617 issued Aug. 20, 1996, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The use of finite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,731,848 issued Mar. 24, 1998, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The design of receivers for both VSB and QAM signals in which both types of signal are processed through the same intermediate-frequency amplifiers receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 to C. B. Patel et alii and entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER is incorporated herein by reference. The detection of data segment synchronization code groups in an HDTV receiver is described by J. Yang in U.S. Pat. No. 5,594,506 issued Jan. 14, 1977, entitled LINE SYNC DETECTOR FOR DIGITAL TELEVISION RECEIVER, and incorporated herein by reference. In U.S. Pat. No. 5,511,099 issued Apr. 23, 1996, entitled PASSBAND SYNC BLOCK RECOVERY, and incorporated herein by reference J. W. Ko et alii describe the use of match filters for detecting prescribed digital sequences having high autocorrelation properties that modulate a radio-frequency carrier in digital recording. These patents are all assigned to Samsung Electronics, Co., Ltd. pursuant to employee invention agreements already in force at the time the inventions disclosed in these patents were made.
In the radio receivers described in U.S. Pat. No. 5,506,636 the final IF signal is digitized and synchrodyne procedures to obtain baseband samples are carried out in the digital regime. In radio receivers that are to have the capability of receiving digital TV signals no matter whether they are transmitted using VSB or QAM, conversion of the signals to final IF signals just above baseband permits the frequencies of the local oscillators in the tuner to remain the same no matter whether VSB or QAM transmissions are being received. The differences in carrier frequency location within the channel are accommodated in the synchrodyning procedures carried out in the digital regime.
A problem that is encountered in the design of digital TV receivers is acquiring carrier synchronization, then symbol synchronization, and then data line and field synchronization rapidly enough that tuning across the band can be done without having to pause overlong at each channel to determine whether it carries programming and what the nature of the programming is. In prior digital TV receiver designs carrier synchronization and symbol synchronization are necessary before data line and field synchronization can be undertaken. The problem of carrier synchronization is considerably more difficult when receiving QAM transmissions, in which there is no accompanying pilot carrier, than it is when receiving VSB transmissions, in which there is an accompanying pilot carrier. In any case carrier synchronization takes some time to complete, and symbol synchronization carried out after synchronous detection takes additional time to complete. The time required for these procedures to be completed can make tuning from channel to channel sluggish. This is particularly so if an automatic successive-channel scan is being conducted by the HDTV receiver, looking for a desired program identification code.
In U.S. Pat. No. 5,511,099 J. W. Ko et alii describe the use of match filters for detecting prescribed digital sequences having high autocorrelation properties that modulate a radio-frequency carrier. The invention is described with particular regard to a digital VCR using a modulated radio-frequency carrier with upper-and lower-frequency sidebands (e.g., a 16-state QAM radio-frequency carrier), though U.S. Pat. No. 5,511,099 indicates the invention may also have application in other fields. The invention is described in terms of each sync block in the recorded information including a short prescribed digital sequence having high autocorrelation properties such as a Barker code or a pseudo-random (PR) sequence (also referred to as a xe2x80x9cpseudo-random noise sequencexe2x80x9d or xe2x80x9cPN sequencexe2x80x9d) inserted as a sync signal into a predetermined time portion (in usual designs, the beginning) of the sync sub-block, or used repeatedly in selected polarities for coding at least a portion of the synchronizing information. The prescribed sequence for the sync information is constructed so as to have a zero-valued direct component, but also to have a high-valued autocorrelation property. A seven-bit Barker Code is indicated in U.S. Pat. No. 5,511,099 to be preferred, because it is shorter than most PR sequences that might be used. The development of the digital tape recording art for digital TV signals has strongly tended to favor the direct recording of NRZI digital codes on electromagnetic video tape using 24-to-25 modulation, rather than the use of modulated radio-frequency carriers for recording, however.
In the digital TV signals proposed for broadcasting, each data field contains 313 data segments or data lines, and the fields are consecutively numbered modulo-two in order of their occurrence. Each data segment or data line starts with a segment synchronization code group of four symbols having successive values of +S, xe2x88x92S, xe2x88x92S and +S. The value +S is one level below the maximum positive data excursion, and the value xe2x88x92S is one level above the maximum negative data excursion. Each data segment or data line is of 77.3 microseconds duration, and there are 832 symbols per data segment for a symbol rate of about 10 megabits/second. The initial segment of each data field is a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample PR sequence followed by three 63-sample PR sequences. This training signal is transmitted in accordance with a first logic convention in the first segment of each odd-numbered data field and in accordance with a second logic convention in the first segment of each even-numbered data field, the first and second logic conventions being one""s complementary respective to each other. The reference sequence(s) can be analyzed to determine the channel characteristic, and an appropriate equalizing filter can then be implemented.
The data segment synchronization code group or data line synchronization code group of four symbols having successive values of +S, xe2x88x92S. xe2x88x92S and +S used in digital TV broadcast signals does not have particularly high auto-correlation properties that peak in only one phasing. However, the PR sequences in the field synchronization code group included in the initial line of each data field are constructed so as to have high auto-correlation properties that peak in only one phasing. The PR sequences in the field synchronization code group have zero-valued direct component as well; or, if they do not, PR sequences from successive fields can be differentially combined to suppress accompanying direct component. Digital TV receivers can be constructed with complex-input-sample digital filters that are strongly selective for the PR sequences in the field synchronization code group. That is, these digital TV receivers are constructed with match filters responding to the final IF signal being modulated in accordance with the field synchronization code group.
The response of such a match filter permits rapid data field synchronization prior to carrier synchronization and symbol synchronization. Knowledge of when the data field begins permits prediction of when data segments begin and end so that data-segment or data-line synchronization can be keyed or gated, rather than continuous in nature, at the very outset of acquiring a channel. This avoids the likelihood of erroneous data segment lock to four-symbol groups in general digital TV data, which four-symbol groups happen to resemble a data-segment or data-line synchronization code group.
The response of a match filter for the field synchronization code group can facilitate carrier synchronization and symbol synchronization as well, if the final standards for HDTV broadcasting prescribe a fixed phasing of the PR sequences respective to the symbols used during data transmission. This is so even in the case of terrestrial through-the-air HDTV broadcasting, which uses vestigial sideband amplitude modulation. While the full sideband has much higher energy than the pilot carrier during the transmission of the PR sequences, the envelope of the transmitted signal will still exhibit amplitude variations that resemble the PR sequences. Accordingly, the amplitude variations in the envelope can still be detected and subjected to match filtering to determine when a field synchronization code group occurs, even though synchronous detection has not yet been accomplished in the radio receiver. This facilitates rapid determination of data synchronization and symbol synchronization by the radio receiver, since these procedures can begin to be carried forward before synchronous detection has been achieved. This facilitates rapidly tuning between HDTV channels.
When one attempts to understand why the pseudo-random sequences in the data field synchronization information give rise to variations in the envelope of the intermediate-frequency signal that reproduce those sequences when a relatively small pilot carrier is present, the inventors suggest that the suppressed-carrier amplitude modulation conveying those pseudo-random sequences, which modulation is essentially single-sideband in nature, be considered to be a phase-modulated carrier wave. Then, the inventors suggest that the relatively small pilot carrier be viewed as being single-sideband amplitude modulation of that phase-modulated carrier wave. When the relatively small pilot carrier and the relatively large phase-modulated carrier wave are of similar phase, the envelope of the combined signal will exhibit increased amplitude, the inventors note. When the relatively small pilot carrier and the relatively large phase-modulated carrier wave are of dissimilar phase, the envelope of the combined signal will exhibit decreased amplitude, the inventors further note. Accordingly, the inventors point out, the envelope of the intermediate-frequency signal, which is a bandpass transform of the baseband combined signal, will exhibit increased and decreased amplitude in accordance with the positive and negative excursions of the pseudo-random sequences in the data field synchronization information.
A 32-state QAM signal provides sufficient capacity for a single HDTV signal, without having to resort to compression techniques outside MPEG standards, but usually some compression techniques outside MPEG standards are employed to encode the single HDTV signal as a 16-state QAM signal. A prescribed 24-bit word is supplied as data-field indexing information. At the time this specification is written there is no training signal included in the QAM HDTV signal. Also, there is no data line synchronization signal for QAM HDTV transmission, at least not one selected as a standard. This specification presumes that field synchronization coding similar to that used in VSB HDTV is included in a 16-state QAM signal having a baud rate of 5.38*106 symbols per second.
The invention is embodied in a radio receiver for receiving digital TV signals, which radio receiver has a match filter for receiving rectified samples of digitized intermediate-frequency signal and generating a pulse at the beginning of each data field responsive to the amplitude of the intermediate-frequency carrier being modulated in accordance with a field synchronization code group. Amplitude variations in the envelope of the intermediate-frequency carrier are detected and subjected to match filtering, to determine when a field synchronization code group occurs. This is done even if synchronous detection has not yet been accomplished in the radio receiver. Determination of data synchronization and symbol synchronization by the radio receiver is speeded, since these procedures can begin to be carried forward before synchronous detection has been achieved. This facilitates rapidly tuning between HDTV channels.